It is known in the automotive industry to include an exhaust gas treatment system utilizing one or more catalytic units, such as a catalytic converter, diesel oxidation catalyst unit, or selective catalytic reduction catalyst unit to improve the emissions in the exhaust. In such catalytic units, it is common for a catalyst to be carried as a coating on a supporting substrate structure, such as a ceramic substrate having a monolithic structure. Typically, such catalyst carriers are oval or circular in cross section and are often wrapped with a layer of a support or mounting mat that is positioned between the catalyst carrier and the outer housing or shell of the unit to help protect the catalyst carrier from shock and vibrational forces that can be transmitted from the housing to the carrier. Typically, the support or mounting mat is made of a heat resistant and shock absorbing type material, such as a mat of glass fibers or rock wool. These mats have typically been treated with a binder that improves the ability of workers to handle the mat when the mats are cut to size and during wrapping of the mat and assembly of the catalytic units. While such constructions work for their intended purpose, there is always room for improvement.
Traditionally, such constructions have involved a single layer of mat wrapped around the catalyst carrier. The mats in these constructions are formed from rolls of mat material that are first cut into sheets, and then treated with a binder before being die cut to the desired width and length for wrapping. While the process is satisfactory for its intended purpose, it produces a significant amount of scrap from the mat material (up to 30% of yield on average), requires the use of binder because of the handling required for the die cuts mats during manufacturing and assembly and requires that inventories of different part numbers be maintained for each different size and shape of die cut required for each specific catalytic unit design. FIG. 1 is an illustration of this process.
Typically in such constructions, the support mat is compressed between the outer housing or shell of the catalytic unit and the catalyst carrier in order to generate a holding force on the catalyst carrier. However, this can be difficult to maintain accurately because of variabilities in the density of the support mat as it is provided before assembly into such units. One known method of providing the desired assembled density for the support mat is to reduce the size of the housing or shell of the unit after the catalyst carrier and the support mat have been placed inside the shell, with the final outside diameter of the shell being determined based upon the desired assembled density for the support mat.